1. Technical Field
The present invention relates generally to orthotic devices for limiting the motion of a limb. More particularly, the present invention relates to a natural foot orthosis for supporting and controlling the movement of a foot of the wearer of the orthosis and a method of fabricating the orthosis.
2. Background Art
The use of orthotic devices to help support and control the movement of the joints of the lower extremities has been a long established practice. Conventionally, when a person injures or loses motor control of their foot, orthotic devices are often used to brace and rehabilitate the foot. One such condition requiring the use of an orthotic device is when a person is unable to lift a foot, otherwise known as drop foot. Drop foot may be the result of brain damage from a stroke or cardiovascular accident, spinal injury, hereditary motor and sensory neuropathies, or neuromuscular disease. Additionally, any damage to the muscle and nerves which activate the muscle of the neuromuscular system related to the foot may cause drop foot.
The neuromuscular system of the foot comprises the dorsiflexor muscles which are those on the front of the leg and below the knee which lift the foot. The action of lifting the foot from a position substantially aligned with the lower leg to a position substantially perpendicular to the lower leg is known by the term dorsiflexion, while the action of extending the foot from a position substantially perpendicular to the lower leg to a position substantially aligned with the lower leg is known by the term plantar flexion. If the dorsiflexors are weak, a person will be unable to lift the foot as required in the swing phase of a step. The inability to lift the foot may cause a person to trip, since the foot may make contact with the ground during the downward swing of the leg.
In order to prevent drop foot, a type of brace or orthosis known as an Ankle Foot Orthosis (AFO) has been used to compensate for weak dorsiflexors by resisting plantar flexion at heel strike and during swing phase. Until the early 1960's, lower extremity bracing was essentially accomplished using metal, first steel and later the lighter aluminum. In the late 1960's, plastic began to be used for orthotic devices, initially laminates and then, as in the case today, thermoplastics. The most prevalent AFO utilized today is the posterior leaf-spring orthosis, which is typically fabricated by molding sheet plastic over a positive mold of a person's extremity. A posterior leaf-spring orthosis is typically a thermoplastic rigid 90.degree. brace made to fit inside of a shoe. The mobility of an ankle held within the posterior leaf-spring orthosis is determined by the trim lines forming the shape of the orthosis and also by the thickness of the plastic. Another important component determining the mobility of the ankle is the attitude of the extremity when the cast was taken, i.e. dorsi-flexed, neutral, or planter-flexed, as well as the particular shoe to be worn taking into account the heel height.
U.S. Pat. No. 5,370,604 issued to Bernardoni discloses one such example of a posterior leaf-spring AFO which assists a person suffering from drop foot, wherein the AFO is formed form a thermoformed polypropylene material. While polypropylene AFO's have the advantage that they can be mass-manufactured and modified after fabrication to ensure a proper fit, until now AFO's constructed of unreinforced polypropylene were too flexible to provide sufficient dorsiflexion assistance. In order to overcome the flexibility of polypropylene, some polypropylene AFO's incorporated corrugations into their posterior surface and extending downward on their medial and lateral surfaces to add strength and stability to the AFO.
The posterior leaf-spring orthosis, however, has the limitation of restricting normal and desirable ankle and knee motion. For instance, an overzealous attempt to prevent drop foot by increasing the dorsiflexion attitude of the orthosis also has the effect of making the knee joint unstable by inhibiting knee extension. Without changing the attitude of the orthosis, the thickness of the posterior leaf-spring orthosis must be increased to provide the additional support to prevent drop foot. However, since the posterior leaf-spring orthosis fits within the shoe of the wearer, the thickness of the orthosis is limited by the available space between the wearer's foot and the shoe.
Therefore, alternative designs for an orthosis have been developed which were superior to the posterior leaf-spring orthosis in that they prevented drop foot and controlled mild to moderate inversion and eversion, while still allowing more normal ankle and knee articulation. Such alternative designs include spiral and hemi-spiral AFO's. These alternative designs have also been manufactured from thermoformable plastic material to maintain the light weight of the AFO. Although these alternative AFO's are preferred by patients due to their more functional, lighter, and energy saving design, this type of bracing was abandoned by most orthotists due to material failure and breakage problems. Once again, the plastic material used to form these orthotic devices is either to flexible to provide adequate support or too brittle to withstand wear and tear without suffering breakage problems.
Accordingly, there is clearly a need for a lower extremity orthotic device having a lighter, cosmetically superior, and dynamic design which is more durable and does not experience material failure through normal use.